陕西快乐十分电视横屏: 基于mps430电能表的原理设计英文原稿

A system for inverter protection and real-time monitoring Eftichios Koutroulisa, John Chatzakisa, Kostas Kalaitzakisa,*, Stefanos Maniasb, Nicholas C. Voulgarisa aDepartment of Electronic and Computer Engineering, Technical University of Crete, GR-73100 Chania, Greece bDepartment of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece Received 5 September 2002; accepted 4 March 2003 Abstract A real-time system for protecting and monitoring a DC/AC converter has been designed and constructed. The proposed system consists of (a) a hardware protection unit for fast reaction, load protection and inverter fail-safe operation and (b) a microcontroller unit for calculating critical parameters of the inverter operation. The control unit malfunctions have not been investigated in this study. The proposed hardware architecture and sensors form a low-cost and reliable control unit. The experimental results show that the proposed system ensures the inverter protection and fail-safe features. The proposed unit can be used to increase the reliability of any power inverter in AC motor drives, renewable energy systems, etc. or can be incorporated in any UPS system. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Real-time system; DC/AC inverter; Fail-safe; Protection; Microcontroller 1. Introduction DC/AC power converters (inverters) are used today mainly in uninterruptible power supply systems, AC motor drives, induction heating and renewable energy source systems. Their function is to convert a DC input voltage to an AC output voltage of desired amplitude and frequency. The inverter specifi cations are the input and output voltage range, the output voltage frequency and the maximum output power. An inverter is required to: 1. always operate within its strict specifi cations, since the inverter may supply power to sensitive and expensive equipment, 2. fail-safelyincaseofmalfunction,sinceinvertersareoften used in harsh environments to electronics, for example, outdoors in case of renewable energy applications with wide temperature and humidity variations and 3. record the inverter state and inform the supplied equipment and/or the operator about the cause of failure. Considering the inverter protection, the designers usually employ special protection devices and control circuits. The most common form of overcurrent protection is fusing [1], but this method is not always effective because fuses have relatively slow response-time, so additional protective equipment is required, such as crowbar circuits or a di=dt limiting inductance. The DC supply and load-side transients can be suppressed with fi lters, which have the disadvantage of increasing the inverter power losses, cost and weight. Current source inverters (CSI) have an inherent over- current protection capability, since proper design of the DC link inductance can provide protection against overload conditions [2]. Voltage source inverters (VSI) include an L- C fi lter at the output stage thus, in case of an output short- circuit condition, the fi lter inductance limits the output current rising rate [3]. In both preceding cases, the high inductance value leads to inverter size and power losses increase. A commonly used protection circuit is shown in Fig. 1 [4]. The inverter output current, load voltage and fi lter capacitor current are sensed and compared to preset limits. If any of the above quantities exceeds the preset limits, an inhibit signal shuts off the DC power supply. In motor drive applications, the inverters are usually protected only from overloading conditions, using either intrusive current sensing techniques, which measure the DC 0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00134-4 Microelectronics Journal 34 (2003) 823–832 www.elsevier.com/locate/mejo *Corresponding author. Tel.: t30-2821-037233; fax: t30-2821- 037530. E-mail address: [email protected] (K. Kalaitzakis). input current or the load current [5–7] or special motor control algorithm techniques [8–10]. However, the above methods do not fully detect all possible fault conditions, e.g. a DC link capacitor short circuit [11]. The advance of the microcontroller technology has led to the implementation of digital control techniques for controlling and monitoring inverters. The use of a Kalman fi lter for monitoring the magnitude and frequency of a UPS output voltage is proposed in Ref. [12]. Although this method has the advantage of integration of a number of control functions in a single chip, it is not adequate for protection of the inverter from many kinds of faults. If this method is extended to monitor more critical signals, then the system response becomes not fast enough to protect the inverter, while the use of a faster microcon- troller or a digital signal processor (DSP) increases the system cost. Several methods have been proposed for fault detection on an inverter. A diagnostic system for the detection of faults of the power switches using output current sensors in a PWM inverter supplying a synchronous machine is presented in Ref. [13]. It is based on the analysis of the current-vector trajectory and of the instantaneous frequency in faulty mode. An expert-system-based fault diagnosis and monitoring method for a VSI is presented in Ref. [14]. The above-mentioned methods intend to assist the system operator to diagnose the inverter malfunction or damage after its occurrence. In all above methods can be noted that most inverters do not fully fulfi ll the previously stated inverter requirements (steps 1–3). In this paper, the development of a low-cost control unit for protecting and monitoring a DC/AC inverter is presented. The proposed system consists of: (a)a hardware protection unit, which compares the appropriate signals at specifi c points of the inverter circuitry with predefi ned levels, in order to determine the proper system operation, and (b)a microcontroller-based, real-time system, which monitors allcritical parametersof the inverter operation and displays them to the system operator in real-time. In case of malfunction, the hardware protection unit immediately turns-off the inverter ensuring the fail-safe feature, while the microcontroller unit informs the system operator about the malfunction conditions. The microcon- troller unit communicates with a computer through an RS- 232 protocol. The necessary inverter parameters are measured with non-intrusive and non-dissipative sensors so that the inverter operation and specifi cations are not affected. The microcontroller-based implementation is preferred over a faster DSP because of its lower cost. However, a DSP would be a favourable solution in case that additional control functions (i.e. power semiconductors control, advanced battery monitoring algorithms, etc.) are to be digitally implemented. The control unit malfunctions have not been investigated in this study. This paper is organised as follows: the inverter hardware and causes of failure are explained in Section 2; the sensors and actuators required to detect problems on time and force the inverter to shut down are explained in Section 3; the proposed control and monitoring unit is presented in Section 4; the microcontroller algorithm is analysed inSection5,whiletheexperimentalresultsare presented in Section 6. 2. Inverter hardware and causes of failure An inverter general diagram is shown in Fig. 2. A bridge built around IGBTs modulates the DC input voltage to a sinusoidal pulse width modulated wave (SPWM). A low- pass, LC-type fi lter is used to demodulate the SPWM to a sinusoidal waveform, while a power transformer is used to produce the required high voltage, low-distortion output (e.g. 220 V, 50 Hz). Alternatively, the power bridge can be built around power MOSFETs [15], depending on the inverter power capability, the DC input voltage value and the desired effi ciency. The problems that may occur during the inverter operation are the following [3,11]: ? input voltage outside the inverter specifi cations, ? overloading conditions, ? output overvoltage transients, e.g. when connecting or disconnecting motors, ? output short-circuit condition, ? output voltage amplitude and frequency outside the inverter specifi cations, ? high ambient temperature, which changes the power semiconductors characteristics, Fig. 1. An inverter protection circuit. E. Koutroulis et al. / Microelectronics Journal 34 (2003) 823–832824 ? high humidity which may affect the electronic parts behavior and fi nally ? other unexpected factors, e.g. faults on the inverter driving circuit, etc. If any of the above-mentioned problems occurs, the invertermustbeshutdownimmediatelyinordertoprotectthe load and the inverter power conversion stages from destruc- tion,whilethesystemoperatormustbeinformedaccordingly about the problem. The mean time between failures (MTBF) for inverters is of the order of several 10,000 h, [9]. 3. The sensors The position of the sensors on the inverter is shown in Fig. 3. Hall-effect-based sensors are used to measure the DC input current and the AC output current. They have advantages compared to shunt resistors, such as isolation from the main power circuit and independency of their characteristics from dust, humidity and time. Also, they feature wide frequency bandwidth, including DC operation and low temperature variation of their characteristics, so they are ideal for current detection on PWM inverters [16,17]. The operation of the Hall-effect sensors without feedback ensures low power-consumption. But, since they response relatively slow cannot protect effectively the power bridge semiconductors from overcurrent conditions. Thus, an overcurrent protection circuit is developed for the protection of every set of parallel-connected MOSFETs, as shown in Fig. 4(a). Referring to this fi gure, the Q1(IRF530) is the power MOSFET to be protected, while the small- signal MOSFET Q2(BS170) prevents wrong reaction of the protection circuit if high voltage appears at the drain during the power MOSFET turn-off state. Under overcurrent conditions, the following inequality holds: ID·rDS;on· R2 R1t R2 $ VBEe1T where IDand rDS;onare the power MOSFET current and on-state resistance, respectively, while VBEis the transistor Q3base-emitter voltage. Fig. 3. The sensors and actuators on the inverter. Fig. 2. A general diagram of an SPWM single-phase inverter. E. Koutroulis et al. / Microelectronics Journal 34 (2003) 823–832825 The operation of the above-described protection circuit is simulated using the IS-SPICE program. The results are shown in Fig. 4(b), where Y2 is a step change of the Q1 power MOSFET current and Y3 is the protection circuit output voltage. It can be observed that the protection circuit output voltage drops to zero in about 100 ns. If this voltage is used to drive the MOSFET, the turn-off process takes place within about 500 ns, from the occurrence of the overcurrent condition, for this particular MOSFET. Since the power MOSFET peak current capability is much higher than its average rating and the current rise time is further limited by the inverter circuit inductances, this protection circuit is considered adequate for overcurrent protection of the MOSFETs. In cases where the DC input voltage is high, the inverter design can be based on alternative semiconductor devices such as IGBTs or BJTs, Fig. 4. The MOSFET protection circuit: (a) the schematic diagram and (b) the simulated output voltage waveform (Y3) for a step increase of the MOSFET current (Y2). E. Koutroulis et al. / Microelectronics Journal 34 (2003) 823–832826 which are characterised by negative saturation voltage temperature coeffi cient. In such case, the protection circuit described above can be used to measure the voltage developed across a current shunt connected in series with the power switch. An IC instrumentation amplifi er is used to measure the AC output voltage, providing high input-impedance, high common-mode rejection and good temperature stability. A voltage divider followed by a unity-gain isolation amplifi er (voltage follower) is used to measure the DC input voltage, protecting the inverter from malfunctions associated with either the DC input power source or the DC link capacitor. In addition, power semiconductor switches are occasionally subjected to overvoltages during the inverter operation. Such conditions are appropriately treated during the inverter design phase, by employing special circuits (e.g. RC snubbers) depending on the inverter topology requirements. An IC temperature sensor is used to measure the ambient temperature. Its output voltage is proportional to temperature,whileofferinggoodlinearityina wide temperature range with high accuracy. Also, negative temperature coeffi cient (NTC), low-cost thermis- tors are used to monitor the temperature of the inverter power MOSFETs. An electromechanical switch (relay, switch S in Fig. 3) is used to isolate the inverter from the DC input source, in case the input voltage exceeds the maximum limit of the inverter specifi cations. 4. Description of the protection and monitoring unit A block diagram of the detection and protection unit is shown in Fig. 5. The sensors described in Section 3 are used to measure the following parameters during the inverter operation: ? the AC output voltage and current, ? the DC input voltage and current and ? the ambient temperature. The above measurements are interfaced to the micro- controller through its A/D converter channels. The micro- controller calculates the rms output voltage value, the output voltage frequency, the inverter load as a percent of the maximum permitted load, the DC input voltage and the ambient temperature. If a battery is used as the inverter DC input source, the microcontroller checks continuously the charge level of the battery, as well. The inverter DC input current, which is the battery discharging current, is monitored and the battery remaining operating time is estimated. A 2 ￡ 16-character liquid crystal display (LCD) interfaced with the microcontroller informs the operator about the inverter parameter values. Also, the above-mentioned sensor signals are compared with predefi ned thresholds and the results are stored in an external register-set consisting of latches. The output values of the MOSFETs overcurrent protection circuits are also stored in the register-set. During normal operation, all register-set bits are in logic state ‘1’, while in case of Fig. 5. Block diagram of the proposed control unit. E. Koutroulis et al. / Microelectronics Journal 34 (2003) 823–832827 inverter specifi cations violation, the corresponding register- set bits are set to logic ‘0’. Then the hardware protection circuit turns-off the inverter and forwards an interrupt to the microcontroller, causing the check of the register-set bits one by one. The microcontroller parses the n